In semiconductor devices requiring superior heat resistance and high breakdown voltage, semiconductor elements are covered or encapsulated by materials. These materials generally require such heat resistance as to endure heat at a temperature of about 150° C. or higher. In particular, materials (encapsulants) to cover or encapsulate optical elements such as optical semiconductor elements require excellent physical properties such as transparency and flexibility, in addition to the heat resistance. Such encapsulants currently used typically in backlight units of liquid crystal displays are exemplified by epoxy resin materials and silicone resin materials.
Patent Literature (PTL) 1 discloses a synthetic high-molecular compound as a material that is highly resistant to heat and dissipates heat satisfactorily. The synthetic high-molecular compound contains at least one third organosilicon polymer having a molecular weight of from 20000 to 800000. The third organosilicon polymer has been formed by linking at least one first organosilicon polymer with at least one second organosilicon polymer through siloxane bonds. The first organosilicon polymer includes a crosslinked siloxane structure, where the siloxane structure refers to a Si—O—Si bonded structure. The second organosilicon polymer includes a linear, linked siloxane structure. Materials of this type, however, are still lower in physical properties.
PTL 2 discloses a resin composition for optical element encapsulation as a resin composition that has excellent transparency, ultraviolet resistance, and thermal coloration resistance and is used for the encapsulation of an optical element. The resin composition contains, as a resin component, at least one silsesquioxane selected from the group consisting of liquid silsesquioxanes including a cage-like structure, containing an aliphatic carbon-carbon unsaturated bond, and being devoid of H—Si bonds; and liquid silsesquioxanes including a cage-like structure, containing a H—Si bond, and being devoid of aliphatic carbon-carbon unsaturated bonds. Unfortunately, however, the resin composition containing such a cage-like silsesquioxane gives a cured product that is relatively rigid, is poorly flexible, and is susceptible to cracking and/or fracture.
PTL 3 discloses a curable composition that essentially contains an organic compound containing at least two carbon-carbon double bonds per molecule, a compound containing at least two SiH groups per molecule, and a hydrosilylation catalyst, where the carbon-carbon double bonds are reactive with SiH groups. The organic compound is exemplified by triallyl isocyanurate. The compound containing at least two SiH groups per molecule is exemplified by chain and/or cyclic polyorganosiloxanes. Disadvantageously, however, materials of this type are still lower in physical properties such as cracking resistance.